The great proliferation of the automobile in the last sixty years has given rise to a corresponding proliferation of facilities for storing fuel for automobiles. The increasing use of long distance pipe lines facilities for transporting petroleum products as well as the need for distributing petroleum products in urban areas has led to the establishment of fluid storage facilities commonly called tank farms. In such a facility a plurality of tanks are interconnected by a series of pipes comprising a fluid conduit which in turn is interconnected as a terminal or distribution point from a long distance pipe line and is further interconnected with stations for transferring the stored products to over-the-road vehicles for local distribution.
In such facilities, particularly tank farms which terminate the receiving end of long distance high volume pipe lines, it is necessary to monitor and control the level of fluid in each tank as well as to monitor other parameters of the tank including internal pressure and temperature.
In modern petroleum facilities it is particularly important that the systems for monitoring and controlling fluid flow and tank parameters be very reliable since the failure to properly ascertain these quantities can lead to gasoline or other petroleum product overflows which are both expensive and dangerous. Several significant fires at petroleum tank farms have occurred recently in this country under conditions which began with an overflow of a particular tank which was not properly monitoried and controlled.
Heretofore most monitoring of tank parameters, such as internal pressure, temperature of the fluid and fluid level have been monitored by field interface devices (FID)s which include a float actuated level indicator and a resistance temperature device (RTD) for ascertaining fluid level and temperature.
Prior art systems have transmitted analog signals from multiturn potentiometers and analog signals from the RTDs over relatively long cable runs back to a central monitoring location. Due to the resistive losses in the long cable runs and the dependence of these losses upon the length of line from the particular tank to the central location, resolution of such prior art systems was relatively poor.
Furthermore, operation of electromechanical fluid control devices associated with the tanks such as motor operated valves, pumps and the like was accomplished by human response to the monitoring outputs wherein the human operator had to activate a separate set of controls associated with the fluid control devices in order to open or close valves or operate pumps at a particular tank.
More recently float level indicators attached to mechanically drive a shaft connected to a shaft angle encoder have been used to monitor the level of fluid in a storage tank. One prior art system has used a pair of mark and space data lines running to each tank to transmit the output of the shaft angle encoder back to a monitoring central location. Such a system requires a dedicated pair of mark space data lines for each tank and furthermore requires that data decoding be accomplished at the monitoring location.
It has been found through experience in working with multitank fluid storage facilities that it is often desirable to measure the temperature of the fluid at differing levels in the tank depending upon the height of the fluid stored within the tank. The prior art has accomplished such a measurement by providing an array of RTDs vertically spaced within the tank, each of which may be connected, one at a time, to the circuit carrying the temperature information back to a monitor. Prior art systems have used float actuated trip switches which mechanically throw a set of switches to connect the appropriate RTD to the data transmission line as the level of fluid in the tank changes. Such a system has the drawback of the inherent lack of reliability associated with a mechanical device as well as the inability to select a working RTD should the RTD selected by the float arrangement fail.
More recently remote storage tank gauging system have been introduced which include apparatus at each tank for performing an analog to digital conversion of the output of an RTD and for transmitting the digital output back to a monitoring location.
Heretofore the control and monitoring of the status of electromechanical fluid control devices such as pumps and motor operated valves has been actuated either locally at each tank or at certain sets of tanks within a larger tank farm or has been accomplished at separate control stations at the remote location where the monitoring of tank parameters takes place.
One of the reasons for the separation between the monitoring systems and the fluid control device systems of prior art remote storage tank facilities has been the difference in the power supply requirements for these systems. Conventionally the fluid control devices are operated through systems including 24 volt or 48 volt relays and the power supplies associated therewith have been of too high a voltage or too unstable to derive proper supply voltages for digital circuitry. Heretofore the prior art has not provided a combined monitoring and control apparatus for use in a multitank fluid storage facility which at one control point both integrates the functions of monitoring tank parameters and controlling fluid control devices. Furthermore, prior to the present invention, it has not been known to interface these devices without creating power supply problems and also to provide immunity from environmental transients and noise, including lightning strikes near the storage facility.